In a middle portion of an intake passage of an engine, an air cleaner for removing a foreign substance such as a dirt or a dust contained in air (intake air) is mounted. For example, Patent Literature 1 discloses an air cleaner which includes: a cleaner case having an inlet duct and an outlet duct; and an air filter retained in the inside of the cleaner case. Air is introduced into a primary side space of the cleaner case from the inlet duct, and the air is moved to a secondary side space through the air filter to be discharged from the outlet duct.
An air flow sensor is arranged at a position downstream side of the air filter (secondary side passage) in the intake passage. An air flow rate is detected by the air flow sensor. Although various kinds of air flow sensors are known as the air flow sensor to be used in such a purpose, recently, a hot wire type air flow sensor has been relatively frequently used from a viewpoint that such an air flow sensor has small air flow resistance. The hot wire type air flow sensor includes a heat line (hot wire) made of platinum, and outputs a voltage which corresponds to a temperature of the hot wire which changes along with a change of a flow speed of air.
As described above, the hot wire type air flow sensor outputs a voltage which corresponds to a change in temperature of the hot wire. Accordingly, to enable the air flow sensor to stably detect an air (intake air) flow rate with high accuracy, it is important to make the flow of air directed to the hot wire stable. In view of the above, including the above-mentioned Patent Literature 1, a conventional air cleaner of this type includes a flow straightening slit, a flow straightening plate or the like in a cleaner case for making the flow of air stable.
However, when the air cleaner includes the flow straightening slit, the flow straightening slit becomes air flow resistance and hence, an output/torque of an engine is lowered. Further, in the case where the flow straightening plate is formed on the cleaner case by molding, there is a concern that a drawback such as worsening of external appearance due to a molding defect (generation of fillets) or lowering of a yield is brought about.